Film Preservation 101 is an occasional series in which we answer our most frequently asked questions.
You may have heard that old films can be dangerous, and potentially even explosive (we covered this topic in Film Preservation 101: Is Nitrate Film Really Dangerous?) and you’re worried about your grandfather’s home movies that you keep in the basement. The good news is that home movie stock was always made with safety film, which has an acetate cellulose base and isn’t flammable. The bad news is that if you’re keeping those relics of family history in your basement, you may find that when you pull them out, there’s a strong smell of vinegar in the can. What’s going on and what can you do?
What is vinegar syndrome?
Early on, Kodak and other companies that made film stock knew that nitrate film was flammable and potentially dangerous. For a number of (mostly business-related) reasons, the full transition to safety film wasn’t made until the early 1950s. It wasn’t long before archivists and other people who handled film noticed that the acetate base was deteriorating and off-gassing acetic acid, a component of vinegar. Harold Brown, an archivist at the British Film Institute’s National Film Archive, coined the term “vinegar syndrome” to describe not just the smell, but the accompanying catastrophic deterioration that occurs when the film base breaks down. The three main enemies of acetate film are heat, moisture, and acid. Any combination of the three will accelerate a film’s demise.
If you encounter films that smell like vinegar, consider it a warning that you need to act. It’s possible that the film is still in really great physical condition. As the film base deteriorates, however, the integrity of the object is compromised and it becomes increasingly difficult to access the images recorded there. What starts with an unpleasant smell will turn into high shrinkage, a separation between the base (the plastic carrier) and the emulsion (the actual images), softening that leads to increased damage, brittleness, and a fused “hockey puck” condition. As a film advances through the stages of vinegar syndrome, it becomes impossible to watch the content on a projector or flatbed viewer without damaging it. Once it has gone too far, even the gentlest of preservation scanners may be unable to make a new copy.
In the Motion Picture Preservation Lab, we monitor the chemical deterioration of acetate films with AD (acid-detecting) strips. Every film we inspect has a strip added to the can as part of the process. We check it a few days later and record the level of acid deterioration based on how far the strip has changed from blue to yellow, or from 0 to 3 (this is similar to any basic pH experiment you may have done in chemistry class but the strips were specially developed for this purpose). A reading of 1.5 is considered the autocatalytic point, at which film can deteriorate at an exponential rate and immediate action is required.
How can vinegar syndrome be prevented?
As with preventing color fading and preserving nitrate, the answer to preventing vinegar syndrome is proper storage. Keeping acetate films cold and dry slows chemical processes and largely prevents vinegar syndrome from taking hold. Even a film with level 2 or 3 vinegar syndrome can be kept in good condition for decades if stored correctly. Slowing down the process of chemical deterioration means that physical deterioration will also slow and buy more time for reformatting if necessary.
How do we fix vinegar syndrome?
This one’s tough, because the answer is we can’t, really. Keeping films cold and dry just buys time. Some institutions have used little packets called molecular sieves to absorb the acetic acid in the enclosed environment of the film can, but the molecular sieves need to replaced periodically, which is costly and labor-intensive. At NARA, we have more than half a million film reels. Proper storage is our best and only solution (and to re-iterate, it works really well!). For smaller institutions or personal collections without access to ideal storage conditions, molecular sieves can be monitored more closely and might help. Just getting those films out of basements and attics and into a main floor closet will go a long way, as well.
When chemical degradation has advanced to physical deterioration (warping, extreme shrinkage, and soft or brittle condition), we need to act quickly to reformat and preserve as much visual information as possible. Some films are so brittle that they cannot be run on even the gentlest of scanners. Some labs will perform a process called “replasticization” which attempts to put some of the moisture and plasticizers back into the film. This is risky because it might not work, and it only gives you one shot on a film scanner. If you set it up wrong, there’s no going back. In those cases, the best solution is sometimes to just keep it frozen and hope that better technology will come in the future.
For more information on vinegar syndrome and how to care for your films, see:
IPI Storage Guide for Acetate Film–published in 1993 but still a comprehensive guide to what vinegar syndrome is and the best way to prevent it
The Home Film Preservation Guide–Chapter 8 covers storage, including how to prepare small film collections for freezing
The Center for Home Movies has solid guidance on caring for home movie collections.